1. Field of the Invention
The present invention is directed to a method of thermoforming a thin, flexible film. More specifically, the present invention is directed to thermoforming a film using the application of sequential vacuums to avoid trapped air pockets that produce surface defects in the film.
2. Description of the Related Arts
It is known to manufacture a thermoformed film using a forming mandrel and a hinged frame. British patent application No. GB2187132 teaches draping a flat sheet over a mold having a face and two sides and then manipulating a frame to wrap the sheet along the mold sides. The frame may be manipulated before or after contacting the sheet with the mold. A vacuum is applied to the film through apertures in the mold surfaces to cause the sheet to conform to the mold. The vacuum is applied simultaneously to both the face and sides of the mold. When the film seals around the mold, the vacuum causes the film to be drawn against the mold and conform to the mold surface. The vacuum fixes the film in position and prevents further stretching. The early application of vacuum before the film can be completely stretched around the mold causes the film to stick to the mold surface before it can be stretched over the mold sides. This causes small pockets of air to be trapped between the film and the mold in areas that lack vacuum apertures. These air pockets cause the film to display an irregular surface appearance.
U.S. Pat. No. 4,975,236 ('236) recognized the problem of surface defects and explained them as "cold-flow lines". The cold-flow lines, better called chill lines, occur when the hot pliable film contacts the cooler mold surface. A portion of the film cools and sticks to the mold surface. As the remaining portion of the film contacts the mold, lines form between the cooled portion and the remaining portion (chill lines). The early application of the vacuum exacerbates this problem. Portions of the film that are not in contact with the mold surface continue to stretch and become thinner and cause an irregular surface appearance in the film because the areas that contact the mold first are thicker than areas that contact the mold later.
The '236 patent attempted to overcome these chill lines by manipulating the frame that holds the film independently of the mold movement. This enabled the film to be "pre-shaped" to conform to the mold prior to contacting the mold and film. In the embodiment described, the mold is U-shaped. The sides of the frame holding the film are moved upwards to form a U-shape that conforms the film to the mold. After the film is pre-shaped, it is contacted with the mold and a vacuum is applied between the mold and film to cause the film to conform to the mold shape. Because the film contacts both the face and sides of the mold substantially simultaneously, portions of the film will stick to the outermost region of the mold surface. A vacuum is applied only after the film contacts the mold (column 4, lines 45-48). This allowed the film to stretch over the mold, but it did not eliminate the trapped air pockets. It also did not enable a portion of the film to be fixed in position while the remainder of the film is stretched over the mold.
Both the British application and the '236 patent are directed to making relatively thick parts by pre-shaping the film before it contacts the mold. This pre-shaping does not reduce the effects of chill lines. The British application is directed to making refrigerator liners and the '236 patent is directed to making a truck canopy. These films are generally thicker than 3.0 mm and are less sensitive to displaying chill lines and trapped air pockets. Thicker films lose less heat and therefore cool more slowly than thin films. Thin films cool much more quickly when they contact the mold wall. This rapid cooling produces more pronounced chill lines. Thicker films have a small percentage difference between thicker and thinner areas. Thinner films have a very large percentage difference between thicker and thinner areas. The separation between these areas becomes more visible in thin films. Visible surface imperfections such as chill lines, trapped air pockets or surface defects all mar the finished article. This becomes especially important when the article must display a very high degree of gloss and distinction of image as needed for exterior automotive applications.
The present invention is directed to thermoforming relatively thin films between 0.2 and 1.0 mm. Thermoformed thin films having a glossy surface are more sensitive to displaying imperfections caused by chill-lines and air pockets. These and other disadvantages of the prior art are overcome by the present invention.